1. Field of the Invention
The invention in general relates to the fabrication of layered superlattice materials, and more particularly to fabrication processes that provide highpolarizability and low fatigue ferroelectric integrated circuit devices and low-leakage current high dielectric constant integrated circuit devices using low processing temperatures.
2. Statement of the Problem
It has been well-known for at least 30 years that if a memory utilizing the polarizability property of ferroelectric materials could be made, such a memory would be non-volatile, of high density, and have many other advantages. See, for example, U.S. Pat. No. 5,046,043 issued to William D. Miller et al. Moreover, it is also well-known that the substitution of high dielectric constant materials for the silicon dioxide of conventional memories such as DRAM's could result in memories that were much more dense. See, for example, European patent application Serial No. 0 415 751 A1 of NEC Corporation. Thus, a large amount of research has been performed for many years to obtain materials with suitable ferroelectric properties and suitable high dielectric constant properties. However, up to the time of the above-mentioned patent copending applications, no one had been able to find a material that had ferroelectric properties or high dielectric properties that made it suitable for fabricating a practical ferroelectric memory or dielectric memory with a suitably high dielectric constant. All ferroelectric materials with suitably high polarizabilities fatigued, and all dielectric materials with suitably high dielectric constant had excessive leakage currents. The above-mentioned copending patent applications disclose that layered superlattice materials, such as strontium bismuth tantalate, have excellent properties in ferroelectric and high dielectric constant applications as compared to the best prior materials, such as PZT. The capacitor memory designs disclosed in the above copending applications, usually included platinum electrodes.
It is known that platinum adheres to silicon only with difficulty, and that a titanium layer placed between a platinum electrode and a silicon substrate will significantly increase the adhesion of the platinum to the substrate. Thus, practical memory designs that can be manufactured using layered superlattice materials and platinum electrodes, generally include an adhesion layer.
The above applications disclose that annealing temperatures of about 800.degree. C. are required to obtain the best electrical properties, such as polarizability greater than about 15 microcoulombs/cm.sup.2. While temperatures of 800.degree. C. were lower than temperatures of the prior art used to make such materials, there still remained some atomic migration through boundaries, like electrodes, at this temperature. For example, titanium used as adhesion layers in electrodes migrated to the ferroelectric material and to the silicon. This atomic migration sometimes changed contact resistances and other properties, thus making it difficult to use the layered superlattice materials with transistors and other conventional electrical components made with conventional silicon technology.